Assessing tobacco use in an African population: serum and urine cotinine cut-offs from South Africa

- Ware, Lisa J.; Charlton, Karen; Kruger, Ruan; Breet, Yolandi; Van Rooyen, Johannes; Huisman, Hugo; Botha, Shani; Uys, Aletta S.; Rennie, Kirsten L.; Naidoo, Nirmala; Kowal, Paul; Schutte, Aletta E.